Spectroscopic Enlightening of the Local Structure Of VOX Active Sites in Catalysts for the Odh of Propane

Site isolation of V active sites has been often correlated to catalytic performance for the oxidative dehydrogenation (ODH) of propane to propylene. In particular, catalyst selectivity seems favored by high V dispersion. The latter property is hardly attainable by traditional preparation methods, especially by impregnation except at very low V loading, which however may lead to a too high surface exposure of the acidic sites of the support. In this paper, the effect of the preparation procedure on catalyst properties has been investigated, particularly considering catalysts prepared by flame pyrolysis, a synthesis method which induced a very high V dispersion also at relatively high vanadium loading. Transmission electron microscopy also allowed us to assess V oxide dispersion depending on both the support type and the preparation method. Furthermore, the local structure of the V active sites has been deeply investigated by X-ray absorption spectroscopy, allowing us to propose a possible structure of the active sites. The average oxidation state of surface V species was then studied by X-ray photoelectron spectroscopy (XPS), showing a role of V oxidation state on catalyst selectivity. The catalytic performance has been interpreted on the basis of V species and catalyst acidity (as measured by IR spectroscopy), another fundamental parameter that in turn results to be correlated with V dispersion on different supports. More selective catalysts were indeed characterized by the presence of weaker Brønsted acidic sites.